The definition of repeated DNA sequences currently used in research is less than adequate. We propose to investigate various aspects of the relationships among repeated DNA sequences in many species of plants and animals so as to provide a more quantitative definition of the phenomenon. Without this basic analysis it seems unlikely that we will understand the putative role of repeated sequences in genome organization, gene regulation, chromosome structure, evolution or any other process. We will analyze DNA reassociation kinetics at several temperatures in order to study the complexity of families of repeated sequences and the degree of similarity of members within families. We propose to investigate patterns of interspersion of repeated and non-repeated sequences by two methods. We will use a previously developed technique of examining reassociated structures in the electron microscope. We will also continue to develop a relatively rapid technique of DNA network, or aggregate formation so as to apply it to interspersion analysis. Some plant satellite DNAs have a large kinetic complexity (coding potential), hybridize with RNA and are therefore basically different from the known animal satellites. We propose to investigate the type of RNA which hybridizes and to compare the sequence arrangement in the plant satellites with that for mouse satellite by electron microscopy and gel electrophoresis. The information we obtain concerning repeated sequences and genome organization will be used in trying to trace the evolution of land plants from green algae to flowering forms.